1. Field of the Invention.
This invention relates to a device for producing a leno selvedge, in particular for a loom, including an electromotor with a rotor, whereas the rotor has at least two spaced guide elements for the leno threads.
2. Description of the Prior Art.
A device for producing a leno selvedge is known out of FR-A 23 90 524, this device having arms being elastic and flexible in axial direction. This device is a so-called "planet rotator." Due to its design, such a planet rotator has a high mass, still increased by the fact that this known planet rotator has the spools for the leno threads arranged directly on the rotator. That means that such a device has a mass so big that it is physically impossible to accelerate and slow down in short time intervals.
The same applies to DE-A 15 35 579 which also shows a so-called planet rotator. This device too is incapable of fast acceleration and fast slowing down due to the movable mass. No other evaluation can be made of U.S. Pat. No. 2,918,092 since the device also associates the yarn spools with its movement.
A device as mentioned above is known out of DE-PS 44 05 776. Hereby, an electrically drivable servomotor is provided which drives a doup disc, the doup disc forming the rotor of the electrically drivable servomotor. The stator itself is mountable onto the loom by means of a carrying element, preferably in a free space between the longitudinal braces and the healds in front to the first heald frames of the loom. More particularly the doup disc, which forms the rotor of the electrically drivable servomotor, has two opposite openings through which one leno thread at a time is led, these openings serving as guide elements. This known rotator for producing a leno selvedge in a loom works so that it completes several hundred revolutions in one direction and ties off one weft thread after each revolution. A full leno selvedge is thus achieved. On the feeding side of the two leno threads, the leno threads are twisted according to the number of revolutions of the doup disc so that, in order to undo this twisting, an inversion of the douping direction is indicated, the douped disc having to rotate in exactly the opposite direction. The number of revolutions in either direction has hereby to be the same in an average period of time. If the douping direction were not inverted, the leno threads would, some time or other, tear due to the increasing tension occasioned by the growing twisting.
In case of the known fast-running looms making up to 1200 werfts per minute, it was ascertained that after approximately 100 to 1000 revolutions in one direction an inversion of the douping direction should occur. That means that, according to the state of the art, the rotor and thus the douping disc of the electrically drivable servomotor has to be reversed every 10 to 100 second. Due to the number of strokes of a loom, the time available for inversion is of 100 millliseconds (msec) maximum. That means that the rotor of the motor has to stand still and to rotate in the reversed douping direction at full speed within 100 msec. Due to the high mass of the douping disc and of the rotor respectively, this performance can be achieved with known motors only at high cost. The idea to reduce the moved mass of the motor by choosing a rightaway smaller motor will naturally arise. The danger incurred in this case is that, if the rotor and thus the douping disc are given a smaller diameter, the shedding occurring between the two leno threads guided through the douping disc is not sufficient, so that the weft thread cannot be inserted accurately. Moreover, the threads can still stick together, impeding the formation of a clean selvedge.
Moreover, during the rotation of the guide elements, the thread tension of the leno threads guided through the guide elements fluctuates. The fluctuations depend on the thread length of the leno threads, themselves depending on the douping angle and on the coning angle of the device relative to the loom. The fluctuations in the thread tension may bring sensitive yarns to tear.